The invention relates to an actuating apparatus with automatic clearance adjustment for a vehicle hydraulic disk brake according to the preamble of claim 1. Such an actuating apparatus is known from EP 0 403 635 B1.
Actuating apparatuses of said type are used in vehicle disk brakes which are to be usable also as an immobilizing brake (parking brake). For said reason, in addition to the hydraulic actuating facility a usually mechanical brake-actuating facility is provided, which comprises a preferably automatically operating clearance adjusting device so that, even when the brake linings are becoming worn, the actuating travel required for mechanical tightening of the brake does not become unduly long. In the case of the initially mentioned actuating apparatus, said clearance adjusting device comprises a first, substantially sleeve-shaped strut part, into which a second strut part having a bolt-shaped end is screwed. Both strut parts therefore form a length-adjustable adjusting strut.
When filling said actuating apparatus with brake fluid, the latter has to pass also into a cavity, which is delimited in the sleeve-shaped strut part at the end face of the latter by a fluid-tight wall and at the opposite other end by the bolt-shaped end of the second strut part. In the known actuating apparatus, the brake fluid passes into the said cavity through the thread pairing between the bolt-shaped end of the second strut part and the sleeve-shaped first strut part. For said purpose, the thread pairing has a certain axial clearance which allows the brake fluid to pass through.
In principle, in order to keep to a minimum the idle travel which has to be overcome during actuation of the parking brake, as small a thread clearance as possible is desirable. However, this involves the risk of air not being properly evacuable from said cavity when the actuating apparatus is filled with brake fluid so that the cavity is possibly not completely filled with brake fluid, which then leads in turn to an increased idle travel during hydraulic actuation.
The invention is to improve an actuating apparatus of the described type in such a way that despite as small as possilbe a thread clearance of the adjusting strut comprising the first and second strut part, trouble-free, i.e. complete filling of the cavity is possible. The invention is achieved by an actuating apparatus which has the features indicated in claim 1. The sleeve-shaped strut part, which rests with a conical flange against the brake piston, accordingly has in the region of the cavity a plurality of radial bores, which connect the cavity to the outer peripheral surface of the sleeve-shaped strut part. The sleeve-shaped strut part moreover has in its conical flange a plurality of axial through-bores, which are disposed relative to one another in such a way that, in any rotational position of the sleeve-shaped strut part relative to the housing of the actuating apparatus, the radially outer end of at least one radial bore, which is situated above a reference plane extending horizontally through the centre line of the actuating apparatus, lies below at least one part of at least one axial through-bore in the conical flange. Thus, above the fluid level in the actuating apparatus there is always one fluid connection between the cavity and the cylinder bore, in which the brake piston is displaceably disposed, so that, as the actuating apparatus is filled with brake fluid, air may be expelled from the cavity into the cylinder bore and complete filling of the cavity with brake fluid is guaranteed. The flow of brake fluid into the cavity is effected through the axial through-bores and radial bores in the sleeve-shaped strut part, which are situated below the said horizontal reference plane.
Thus, according to the invention, selection of the clearance of the thread pairing between the first and the second strut part need no longer involve having to ensure that brake fluid and/or air may pass through unimpeded. Rather, said thread clearance may be optimized in the sense of minimum idle travel during mechanical actuation.
In a preferred embodiment of the actuating apparatus according to the invention, in the sleeve-shaped strut part at least three radial bores and three axial through-bores are provided, which are in each case uniformly spaced apart from one another in peripheral direction. In another embodiment, four radial bores and six axial through-bores are provided, which are likewise uniformly distributed in peripheral direction.
Preferably, the numerical value of the ratio of the volume of the said cavity to the cross-sectional area of each axial through-bore and also the numerical value of the ratio of the volume of the cavity to the cross-sectional area of each radial bore is not greater than 1000. At any rate, the cross-sectional area of each radial bore and of each axial through-bore has to be large enough to prevent the corresponding opening from being blocked by drops of fluid which form as a result of the surface tension of the utilized fluid. If the numerical value of the said ratios is xe2x89xa61000, then the said cavity may be completely filled with brake fluid within a reasonable time even at a relatively low filling pressure, e.g. a pressure of up to 6 bar.
When the actuating apparatus according to the invention comprises a spring washer arrangement, which is supported axially between a retaining ring and the conical flange of the sleeve-shaped strut part, then the retaining ring preferably has a cross section, which impedes the flow of brake fluid into the cavity and of air out of the cavity as little as possible and may, for example, be round. The spring washer arrangement, which in the described prior art is used together with a retaining ring of rectangular cross section, normally holds the conical flange against the corresponding inner cone of the brake piston and thereby prevents a rotation of the sleeve-shaped strut part relative to the brake piston. When the actuating apparatus according to the invention comprises a restoring spring, which is clamped in between the expanding device required for mechanical application of the brake piston and an approximately pot-shaped spring cage, then said spring cage preferably has in the region of its base a plurality of through-openings in order to impede a throughflow of brake fluid and air as little as possible. The said restoring spring is used to preload the expanding device into its normal position.
By virtue of the actuating apparatus according to the invention an improved vehicle disk brake with an immobilizing function is also provided, the idle travel of which during actuation of the mechanical immobilizing function may be kept very low.